Volumetric flow rates at which gases travel through closed pipes are at times measured by placing a flowmeter directly in the pipe flow path. When relatively low levels of flow, e.g., two to fifty cubic feet per minute (CFM), are monitored in relatively large pipes, e.g., pipes six or more inches in diameter, the low level flow may be bypassed through a smaller pipe in order to increase the velocity of fluid flow and thereby improve the accuracy of measurement. For example, air inleakage in steam turbines is actively exhausted in order to minimize corrosion of turbine components and to reduce vibration in low pressure turbine blading. Efforts are made to keep air inleakage rates below ten CFM during normal operating conditions, but rates may range up to 400 CFM during turbine start up. High compression ratio pump designed to exhaust this wide range of flow must operate under low back pressure. Otherwise, excessive exhaust back pressures may damage the pump seals. Consequently, the vent pipes which exhaust this air at least six inches in diameter and may be larger for long pipe lengths in order to minimize rises in back pressure when the pumps displace large volumes of air.
Flowmeters which have been used for measuring volumetric exhaust rates in these vent pipes have required a minimum flow velocity of approximately 50 feet per minute in order to maintain an acceptable level of accuracy. However, the velocity of a one CFM flow through a six inch pipe is on the order of only five feet per minute. Therefore it has been necessary to bypass turbine exhaust air through a flow monitor having a markedly smaller inside diameter than the vent pipe in order to bring the exhaust air velocity into an acceptable range for measuring volumetric flow rates. When a flowmeter is connected in parallel with the vent pipe, the bypass valve used to divert exhaust flow to the flowmeter must completely seal off the vent seal in order to sustain accurate measurements. During normal low flow operations the valve remains closed in order to effect continuous monitoring. When the exhaust flow rate increases, e.g., due to a sudden inleakage of air to the turbine system, the bypass valve must be quickly opened in order to avoid excessive back pressure which would result from moving the increased volume of gas through the relatively small diameter flowmeter pipe.
Valves which have been used in the past for bypassing exhaust air to a flowmeter are believed to have several undesirable limitations. For example, wear along metallic closing surfaces of conventional stop valves may result in a failure to create a tight seal. The resulting leakage through the stop valve may escape detection and result in spurious flow data. Secondly, valves of the type which have been used in the past for bypassing exhaust air are known to occasionally stick in a closed position, thereby presenting a potential cause of damage to the turbine system. A fail-safe open bypass valve would provide the fullest protection to the turbine blading and the pump seals when back pressure increases. It is therefore desirable to have a bypass valve which provides a seal when closed and which reliably vents exhaust gas when back pressure exceeds a predetermined safe value.